A dominant issue in the health reform debate is whether insurance coverage should be voluntary or mandatory. Clearly, the factors that determine who will seek voluntary coverage are relevant to this policy issue. This article uses experience from Washington State’s Basic Health Plan to examine the enrollment choices of low-income families in a state-subsidized voluntary insurance plan offered through managed care organizations. We hypothesize that the decision to enroll, which encompasses the decisions to purchase insurance coverage and to select a particular plan, is influenced by four factors: the family’s financial vulnerability, their risk perception, the price of coverage, and the transition costs of enrolling. Our enrollment model is supported by the data and has important implications for the design of voluntary programs. Families who choose to enroll are more likely to have a female head of household, young children, and a family member who has a part-time job and some college education. Higher premiums and availability of other insurance coverage decrease the probability of enrolling.